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RF Power Amplifier Design

VIEWS: 29 PAGES: 27

� Basic Amplifier Concepts l Class A, B, C, F, hHCA l Linearity Aspects l Amplifier Example � Enhanced Amplifier Concepts l Feedback, Feedforward, ... l Predistortion l LINC, Doherty, EER, ...

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									RF Power Amplifier Design




                              Markus Mayer & Holger Arthaber
                         Department of Electrical Measurements and Circuit Design
                                                  Vienna University of Technology

                                                                   June 11, 2001




Contents

¤ Basic Amplifier Concepts
   l Class A, B, C, F, hHCA
   l Linearity Aspects
   l Amplifier Example

¤ Enhanced Amplifier Concepts
   l Feedback, Feedforward, ...
   l Predistortion
   l LINC, Doherty, EER, ...

                                                                                    2
Efficiency Definitions


                                            POUT
¤ Drain Efficiency:                 ηD =
                                            PDC



¤ Power Added Efficiency: η PA = POUT − PIN = η D ⋅ 1 − 1 
                                                          
                                    PDC              G




                                                                  3




Ideal FET Input and Output Characteristics

                    IDS
                                   VGS=0
                    Im



           gm

                                VGS=VP
                     VGS                                    VDS
2VP   VP        0          0   VK      VDD         VDSmax

                           Ohmic      Saturation   Breakdown



                                           VDD − V K
                                      κ=
                                             VDD

                                                                  4
Maximum Output Power Match
                    I DS
                                    VGS=0
                    Im



           gm

                                VGS=VP
                     VGS                                        VDS
2VP   VP        0          0   VK      VDD             VDSmax

                           Ohmic       Saturation      Breakdown



                                            VDS max − VK
                               ROPT =
                                                 Im

                                                                                         5




Class A
                    IDS                                          IDS

                    Im                                             Im




                     VGS                                        VDS                  Q
2VP   VP        0          0   VK           VDD     VDSmax              0   p   2p
                     VGS                                        VDS



                     p



                    2p
                     Q

                                                                                         6
Class A – Circuit
                                    VDD




                                G    D
                                                    RL
                                     S




                               η D = κ ⋅ 50%

                               G = G A (e.g. 14 dB)

                               η PA = κ ⋅ 48%

                                                                                 7




Class B
               IDS                                        IDS

               Im                                          Im




                VGS                                      VDS                 Q
2VP   VP   0          0   VK        VDD         VDSmax          0   p   2p
                VGS                                      VDS



               p



               2p
               Q

                                                                                 8
Class C
               IDS                                      IDS

               Im                                       Im




                VGS                                   VDS                  Q
2VP   VP   0          0   VK          VDD   VDSmax            0   p   2p
                VGS                                   VDS



               p



               2p
               Q

                                                                                9




Class B and C – Circuit
                                VDD


                                             f0

                            G    D
                                                       RL
                                 S




                     Class B                         Class C
                     ηD = κ ⋅ 78%                    ηD → 100%

                     G = G A - 6dB (8 dB)            G →1

                     ηPA = κ ⋅ 65%                   ηPA → 0%
                                                                               10
Influence of Conduction Angle




                                                                      11




Class F (HCA ... harmonic controlled amplifier)
               IDS                             IDS

               Im                               Im




                VGS                           VDS                 Q
2VP   VP   0          0   VK   VDD   VDSmax          0   p   2p
                VGS                           VDS



               p



               2p
               Q

                                                                      12
hHCA       (half sinusoidally driven HCA)

               IDS                                            IDS

               Im                                              Im




                VGS                                          VDS                  Q
2VP   VP   0          0    VK    VDD           VDSmax               0    p   2p
                VGS                                          VDS



               p



               2p
               Q

                                                                                      13




Class F and hHCA – Circuit
                          VDD



                                                 Zo(n)

                                                0, n=1
                            ID    Ze(n)         inf, n=odd

                    VDS          0, n=even
                                                                    RL
                                 inf, n=even




                     Class F                                 hHCA
               ηD = κ ⋅100%                         ηD = κ ⋅100%

               G = G A - 5dB (9 dB)                 G = G A + 1dB (15 dB)

               ηPA = κ ⋅ 87%                        ηPA = κ ⋅ 96%
                                                                                      14
hHCA – Third Harmonic Peaking
               IDS                                          IDS

               Im                                            Im




                VGS                                        VDS                 Q
2VP   VP   0          0    VK         VDD      VDSmax             0   p   2p
                VGS                                        VDS



               p



               2p
               Q

                                                                                   15




Third Harmonic Peaking – Circuit
                          VDD




                      G    D             3f0
                                               f0            RL
                           S




                               ηD = κ ⋅ 91%

                               G = G A + 0.6dB (14.6 dB)

                               ηPA = κ ⋅ 87%

                                                                                   16
  Linearity Aspects




                               17




  Linearity Aspects



¤ Class A         ¤ Class AB




¤ Class B         ¤ Class C




                               18
Linearity Aspects




¤ Ideal strongly nonlinear model   ¤ Strong-weak nonlinear model


                                                                   19




Amplifier Design – An Example
¤ Balanced Amplifier Configuration




  Port 1
  Z=50 Ohm                                           Port 2
                                                  Z=50 Ohm




                                                                   20
Amplifier Design – Simulation
¤ Gate & Drain Waveforms

                                          Gate waveforms                                                                                         Drain waveforms
1                                                                                                      1000     25                                    Inner Drain Voltage (L, V)   Inner Drain Current (R, mA)
                                                                                                                                                                                                                    5000
                                                                                                                                                      Amp                          Amp




                                                                                                                20                                                                                                  4000
0                                                                                                      500
                                                                                                                15                                                                                                  3000


-1                                                                                                     0        10                                                                                                  2000


                                                                                                                 5                                                                                                  1000
-2                                                                                                     -500
             Inner Gate Voltage (L, V)         Inner Gate Current (R, mA)
                                                                                                                 0                                                                                                  0
             Amp                               Amp

-3                                                                                                     -1000    -5                                                                                                  -1000
     0                                   500                                1000                1300                 0                       500                                   1000                          1300
                                                Time (ps)                                                                                           Time (ps)




                                                                                                                                                                                                                              21




Amplifier Design – Simulation
¤ Dynamic Load Line & Power Sweep

                                               Dynamic load line                                                                                 Power Sweep 1 Tone
8000                                                                                                            40                                                                                                       80
                                                                              IVCurve (mA)
                                                                                                                         Output Power (L, dBm)
                                                                              IV_Curve                                                                                                                                   70
                                                                                                                         Amp
6000                                                                          Dynamic Load Line (mA)                     PAE (R)
                                                                              Amp                               30       Amp                                                                                             60

4000                                                                                                                                                                                                                     50

                                                                                                                20                                                                                                       40
2000                                                                                                                                                                                                                     30

                                                                                                                10                                                                                                       20
     0
                                                                                                                                                                                                                         10

-2000                                                                                                            0                                                                                                       0
         0                    3                   6                     9             12                   15        0          5                  10           15                                  20              24
                                                      Voltage (V)                                                                                   Power (dBm)




                                                                                                                                                                                                                              22
Amplifier Design – Measurements
¤ Single Tone & Two Tone

                                                                                                                                   60                                                  60 PAE[%]
                    40                                                            A %
                                                                              80 P E[ ]

                                                                    1dBCP
                    35                                                        70
                                                                                                                                   50                                                  50


                    30                                                        60




                                                                                              P out [dBm], IMDD [dBc], Gain [dB]
                                                                                                                                   40                                                  40
P out [dBm], Gain [dB]




                    25                                                        50

                                                                                    Pout                                                                                                      Pout
                                                                                                                                                                                              IMDD
                                                                                    Gain                                           30                                                  30
                    20                                                        40                                                                                                              Gain
                                                                                    GammaIn
                                                                                                                                                                                              PAE
                                                                                     A
                                                                                    PE
                    15                                                        30
                                                                                                                                   20                                                  20


                    10                                                        20

                                                                                                                                   10                                                  10
                         5                                                    10



                         0                                                     0                                                   0                                                    0
                             0   5   10   15              20   25       30   35                                                         0   5   10   15                20   25   30   35
                                               Pin[dBm]                                                                                                   P in [dBm]




                                                                                                                                                                                                     23




Amplifier Nonlinearity
¤ Gain and Phase depends on Input Signal

¤ 3rd Order Gain-Nonlinearities:




                                                                                                                                                                                                     24
Amplifier Nonlinearity
¤ Higher Output Level (close to Saturation) results
  in more Distortion/Nonlinearity




                                                      25




Nonlinearity leads to?
¤ Generation of Harmonics

¤ Intermodulation Distortion / Spectral Regrowth

¤ SNR (NPR) Degradation

¤ Constellation Deformation




                                                      26
Intermodulation and Harmonics




                                                                                              27




Spectral Regrowth
                                10
                                                                           ACPR 1>60dB
                                                                           ACPR 2>60dB
                                 0
                                                                           ACPR 1=16dB
                                                                           ACPR 2=43dB
                                -10
          relative power / dB




                                -20


                                -30


                                -40


                                -50


                                -60
                                   -15   -10   -5            0         5          10     15
                                                relative frequency / MHz


¤ Energy in adjacent Channels
¤ ACPR (Adjacent Channel Leakage Power Ratio) increases

                                                                                              28
Reduced NPR             (Noise Power Ratio)

¤ Input Signal                ¤ Output Signal of
                                Nonlinear Amplifier




¤ Degradation of Inband SNR
¤ „Noisy“ Constellation


                                                                    29




Constellation Deformation
¤ Input Signal                ¤ Output Signal of
                                Nonlinear Amplifier
                                (with Gain- and Phase-Distortion)




                                                                    30
Modeling of Nonlinearities
¤ with Memory-Effects
   l Volterra Series (=„Taylor Series with Memory“)

¤ without Memory-Effects
                            αar                 α Θr 2




                                                          performance
   l Saleh Model f (r ) =            g (r ) =
                          1 + β ar 2          1 + βΘr 2




                                                             better
   l Taylor Series
   l Blum and Jeruchim Model
   l AM/AM- and AM/PM-conversion



                                                                        31




AM/AM- and AM/PM-Conversion
¤ GaAs-PA




                                                                        32
AM/AM- and AM/PM-Conversion
¤ LDMOS-PA




                                         33




How to preserve Linearity?
¤ Backed-Off Operation of PA
   l Simplest Way to achieve Linearity

¤ Linearity improving Concepts
   l Predistortion
   l Feedforward
   l ...




                                         34
How to preserve Efficiency?
¤ Efficiency improving Concepts
   l Doherty
   l Envelope Elimination and Restoration
   l ...

¤ Linearity improving Concepts
   l Higher Linearity at constant Efficiency
     à Higher Efficiency at constant Linearity




                                                 35




Direct (RF) Feedback




¤ Classical Method
¤ Decrease of Gain à Low Efficiency
¤ Feedback needs more Bandwidth than Signal
¤ Stability Problems at high Bandwidths
                                                 36
Distortion Feedback




¤ Feedback of outband Products only
¤ Higher Gain than RF feedback
¤ Stability Problems due to Reverse Loop

                                                      37




Feedforward




¤ Overcomes Stability Problem by forward-only Loops
¤ Critical to Gain/Phase-Imbalances
  0.5dB Gain Error à -31dB Cancellation
  2.5° Phase Error à -27dB Cancellation
¤ Well suited for narrowband application

                                                      38
                 Cartesian Feedback
baseband input
                  I
                              modulator     main amp.
                              I                                                         RF-output
                       OPAs
                              Q
                 Q

                                                local
                                                oscillator
                                                                                   10
                                                                                         UMTS example:
                                                                                                                           original signal
                              I                                                                                            predistorted signal
                              Q                                                      0


                              demodulator                                          -10




                                                             relative power / dB
                                                                                   -20

       ¤ AM/AM- and                                                                -30

         AM/PM-correction                                                          -40

       ¤ High Feedback-Bandwidth                                                   -50


       ¤ Stability Problems                                                        -60
                                                                                      -30      -20   -10          0        10       20           30
                                                                                                     relative frequency / MHz
                                                                                                                                                      39




                 Digital Predistortion
                 ¤ Digital Implementation of „Cartesian Feedback“
                 ¤ Additional ADCs, DSP Power, Oversampling needed
                 ¤ Loop can be opened à no Stability Problems




                                                                                                                                                      40
Analog Predistortion




¤ Predistorter has inverse Function of Amplifier
¤ Leads to infinite Bandwidth (!)
¤ Hard to realize (accuracy)


                                                   41




Analog Predistortion
¤ Possible Realizations:




                                                   42
LINC      (Linear Amplification by Nonlinear Components)

                      s1(t)       Ks1(t)
                              K
                                                                  K(s1(t)+s2(t))
s(t)       signal                                                    =Ks(t)
         separation

                      s2(t)       Ks2(t)
                              K

                                                                       UMTS example:
                                                                 10
¤ AM/AM- and                                                               s(t)
                                                                                                               ACPR 1 >60dB
                                                                                                               ACPR 2 >60dB

  AM/PM-correction                                                 0       s1 (t)
                                                                                                               ACPR 1 =18dB
                                                                                                               ACPR 2 =29dB

¤ Digital separation required
                                                                 -10




                                           relative power / dB
  (accuracy!)                                                    -20


¤ High Bandwidth,                                                -30


  oversampling necessary                                         -40


¤ Stability guaranteed                                           -50


                                                                 -60
                                                                    -30   -20       -10          0        10     20       30
                                                                                    relative frequency / MHz
                                                                                                                               43




Doherty Amplifier
¤ Auxiliary amplifier supports main amplifier during saturation
¤ PAE can be kept high over a 6dB range




                                                                                                                               44
 Doherty Amplifier
 ¤ Gain vs. Input Power                                                       ¤ Efficiency vs. Input Power
POUT



                                               2)
                                             +A
                                          (A1
                                        n
                                     io       main amp. (A1)
                                   at
                                 ur
                            ig
                          nf
                     co
               rty                         aux. amp. (A2)
          he
       do


                                                               PIN

 ¤ No improvement of AM/AM- and AM/PM-distortion
 ¤ Behavior of auxiliary amplifier very hard (impossible) to realize
 ¤ Stability guaranteed

                                                                                                                  45




 EER                       (Envelope Elimination and Restoration)

 ¤ Separating phase and magnitude information
 ¤ Elimination of AM/AM-distortion
 ¤ Application of high-efficient amplifiers
   (independent of amplitude distortion)
 ¤ Stability guaranteed
                                                                     amplitude information

                                 RF input           signal
                                                  separation
                                                                     phase information                RF output

                                                                                    high efficiency
                                                                                    power amplifier


                                                                                                                  46
        EER                          (Envelope Elimination and Restoration)

¤ Analog realization                                                                                               peak detector
                                                                                                                                                          supply voltage
                                                                                                                                                            amplifier

   l Limiter hard to build
   l Accuracy problems                                                                                                                                  limiter
   l Feedback necessary                                                       RF input                                                                                                               RF output


                                                                                                                                                                        high efficiency
                                                                                                                                                    peak detector       power amplifier

¤ Digital realization
   l Oversampling + high D/A-                                                                                                                  amplitude information
      conversion rates required                                                                                                                D
                                                                                                                                                                         supply voltage amplifier




                                                                          digital baseband input
   l High power consumption                                                                        I
                                                                                                                                                    A

      of DSP and D/A-converters                                                                               digital                          D                    modulator
                                                                                                              signal
   l Possible feedback                                                                             Q        processor                               A             I                                 RF output
                                                                                                                                                                  Q
      elimination                                                                                                                              D
                                                                                                                                                                                   high efficiency
   l Compensation of AM/PM-                                                                                                                         A                              power amplifier
                                                                                                                                              phase information
      distortion possible                                                                                                                                                   local oscillator



                                                                                                                                                                                                                47




        EER                          (Envelope Elimination and Restoration)

        ¤ Bandwidth of Magnitude- and                                                                        ¤ Five times (!) oversampling
          phase-signal have higher than                                                                        necessary to achieve standard
          transmit signal                                                                                      requirements

                               UMTS example:                                                                                              UMTS example:
                         10                                                                                                         10
                                                                                                                                              full bandwidth                         ACPR1 >60dB
                                                                    Magnitude
                                                                    Phase                                                                     3⋅B0 bandwidth                         ACPR2 >60dB
                          0                                                                                                          0
                                                                                                                                              5⋅B0 bandwidth                         ACPR1 =33dB
                                                                                                                                              7⋅B0 bandwidth                         ACPR2 =40dB
   relative power / dB




                         -10                                                                                                        -10
                                                                                                              relative power / dB




                                                                                                                                                                                     ACPR1 =51dB
                                                                                                                                                                                     ACPR2 =36dB
                         -20                                                                                                        -20
                                                                                                                                                                                     ACPR1 =53dB
                                                                                                                                                                                     ACPR2 =49dB
                         -30                                                                                                        -30

                         -40                                                                                                        -40

                         -50                                                                                                        -50

                         -60                                                                                                        -60
                            -30    -20   -10          0        10    20                                30                              -30    -20       -10          0        10          20        30
                                         relative frequency / MHz                                                                                       relative frequency / MHz



                                                                                                                                                                                                                48
          Adaptive Bias
          ¤ Varying/Switching of Bias-Voltage depending on
            Input Power Level
          ¤ Selection of Operating Point with high PAE
          ¤ Applicably for nearly each type of Amplifier
                                                                   peak detector
                                                                                        bias
                                                                                       control



                                         RF input                                                   RF output


                                                                                       high efficiency
                                                                                       power amplifier

                                                                                                                       49




          Adaptive Bias
          ¤ Single tone PAE for switched                                            ¤ Simply to implement Concept
            VDD with VG kept constant                                               ¤ Stability guaranteed
                             90                                                     ¤ Possible problems:
                             80                                                        l DC-DC converter with high
power added efficiency / %




                             70                                                          efficiency necessary
                             60                                                        l Possible Linearity Change
                             50
                                                                                         (can increase and decrease)
                             40
                                                                                         especially for HCAs
                                                                        V D =3.5V
                             30                                         V D =4.5V
                                                                        V D =6.5V
                             20
                              32   33   34   35      36     37     38   39     40
                                              output power / dBm




                                                                                                                       50
Summary
¤ Digital Realization required to achieve Accuracy

¤ Problem of Stability for high Bandwidth Application

¤ Higher Bandwidths (Oversampling) necessary,
  depending on Order of IMD cancellation

¤ Predistortion gives best Results while keeping
  Efficiency high (valid for high Output Levels > 40dBm)


                                                           51




Figure References
¤ F. Zavosh et al,
  “Digital Predistortion Techniques for RF Power
  Amplifiers with CDMA Applications”,
  Microwave Journal, Oct. 1999

¤ Peter B. Kenington,
  “High-Linearity RF Amplifier Design”,
  Artech House, 2000

¤ Steve C. Cripps,
  “RF Power Amplifiers for Wireless Communications”,
  Artech House, 1999

                                                           52
Contact Information



  DI Markus Mayer              DI Holger Arthaber

   ( +43-1-58801-35425            ( +43-1-58801-35420



- markus.mayer@tuwien.ac.at   - holger.arthaber@tuwien.ac.at




                                                               53

								
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